1. Field of the Invention
The present invention relates generally to a transmission/reception apparatus and method in a mobile communication system. In particular, the present invention relates to a transmission/reception apparatus and method for providing compatibility between Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) in a forward link.
2. Description of the Related Art
In general, Code Division Multiple Access (CDMA) mobile communication systems, which are divided into synchronous systems and asynchronous systems, provide services using various channels. Channels used in the CDMA mobile communication system are divided into forward channels formed in a direction from a base station to a mobile station and reverse channels formed in a direction from a mobile station to a base station. The forward channels include a pilot channel, a sync channel, a traffic channel, etc.
The existing synchronous CDMA mobile communication standard specifies that N Walsh codes generated using an N×N Hadamard matrix are used as channelization codes for a forward link. For example, for a physical channel defined in a 2nd generation (2G) CDMA mobile communication (IS-95A/B) standard, which is incorporated herein by reference, a total of N=64 Walsh codes are used. Of the 64 Walsh codes, a code W0[1 . . . 1] is used for spreading a pilot channel, a code WN/2=[1 . . . 1−1 . . . −1] is used for spreading a sync channel, and the remaining codes are used for spreading traffic channels. Such a CDMA technique is equally used in most of the conventional CDMA mobile communication systems for the following reasons. The next generation synchronous CDMA mobile communication standard is specified in 3rd Generation Partnership Project 2 (3GPP2), a synchronous CDMA standardization organization, such that it maintains compatibility with IS-95A/B. Therefore, the forward channels should include the pilot channel and the sync channel. The pilot channel is defined in the standard such that with the use of the pilot channel, a mobile station identifies a base station and performs major CDMA functions such as initial synchronization and soft handover. The sync channel supports a function of synchronizing a system time between a mobile station and a base station such that each base station can operate on a synchronous basis. Thus, the sync channel is necessary for CDMA communication. Conventionally, the base station and the mobile station are synchronized with each other through the pilot channel and the sync channel, and thereafter, a voice service and a data service are provided through given Walsh codes and traffic channels.
FIG. 1 is a block diagram illustrating a structure of a transmitter in a conventional CDMA mobile communication system. Referring to FIG. 1, a pilot signal generated by a pilot signal generator 110 for power control is spread in a multiplier 112 by a channelization code for detecting a pilot channel. Commonly, a 0th Walsh code W0 is used as a channelization code for detecting the pilot channel. The pilot signal is defined in the standard such that with the use of the pilot signal, a mobile station identifies a base station and performs major CDMA functions such as initial synchronization and soft handover as well as power control. The pilot channel signal spread by the channelization code is scrambled in a multiplier 114 by a pseudo-random noise (PN) sequence for detecting a base station. A sync signal generated by a sync signal generator 120 for synchronization between a base station and a mobile station is spread in a multiplier 122 by a channelization code for detecting a sync channel. Commonly, an (N/2)th Walsh code WN/2 is used as a channelization code for detecting the sync channel. Here, N denotes the total number of Walsh codes used in the CDMA mobile communication system. For example, when N=64 Walsh codes are used, a 32nd Walsh code W32 is used to distinguish the sync channel. The sync channel signal spread by the channelization code is scrambled in a multiplier 124 by the PN sequence. A traffic signal for transmitting data is generated by a traffic data generator 130, and is spread in a multiplier 132 by any one of the remaining channelization codes except the channelization codes allocated to the pilot channel and the sync channel. Walsh codes Wi(i=1,2, . . . ,N/2−1, N/2+1, . . . ,N−1) can be allocated as channelization codes for the traffic channels. The traffic channel signal spread by the channelization code is scrambled in a multiplier 134 by the PN sequence. Although only one traffic channel is shown in FIG. 1, it should be obvious that a plurality of traffic channels can be used. The pilot channel signal, sync channel signal, and traffic channel signal, all of which were scrambled by the PN sequence, are summed up into one signal in a summer 140. The summed signal is modulated by a modulator 150.
As described above, in the common CDMA mobile communication system, channelization codes for the necessary forward channels are fixed, and the remaining channelization codes are used for transmitting data. That is, in the existing CDMA scheme, a unique Walsh code is allocated to each traffic channel. For example, for a forward fundamental channel (F-FCH), a forward supplemental channel (F-SCH) and a forward packet data channel (F-PDCH), each base station allocates Walsh codes determined by channel resources available at a particular time to each mobile station, and the traffic channels are orthogonally spread by the allocated Walsh codes before being transmitted.
For a high-speed wireless multimedia service targeted by the next generation mobile communication system, wideband spectrum resources are required. However, when wideband spectrum resources are used, fading effects on a wireless transmission path caused by multipath propagation are prominent, and frequency selective fading can be observed even within a transmission band. Therefore, for high-speed wireless multimedia service, an OFDM scheme which is robust against frequency selective fading is superior to a CDMA scheme which uses a spreading gain having a PN sequence and a Walsh code. Thus, currently, active research is being performed on the OFDM scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme using the same.
Generally, the OFDM scheme has excellent spectrum efficiency because spectrums of subchannels are overlapping each other, maintaining mutual orthogonality. In the OFDM scheme, modulation is implemented by Inverse Fast Fourier Transform (IFFT) and demodulation is implemented by Fast Fourier Transform (FFT). As a multiple access scheme based on the OFDM scheme, there is an Orthogonal Frequency Division Multiple Access (OFDMA) scheme in which some of all subcarriers are allocated to a particular user. The OFDMA scheme does not require a spreading sequence for spreading. The OFDMA scheme can dynamically change a set of subcarriers allocated to a particular user according to a fading characteristic of a wireless transmission path, and this is commonly called “dynamic resource allocation” or “frequency hopping.”
Meanwhile, as a multiple access scheme requiring a spreading sequence, there are a spreading-in-time-domain scheme and a spreading-in-frequency-domain scheme. The spreading-in-time-domain scheme spreads a user signal in a time domain, and then maps the spread signal to a subcarrier. The spreading-in-frequency-domain scheme demultiplexes a user signal in a time domain, maps the demultiplexed signal to a subcarrier, and distinguishes the user signal using an orthogonal sequence in a frequency domain.
However, in an environment where the existing synchronous CDMA transmission standard is being extensively serviced, a new standard based on OFDMA has many problems. That is, in a cellular network of FIG. 6 where cells supporting OFDMA technology coexist with cells supporting CDMA technology, service continuity may not be guaranteed due to handover between cells supporting different communication technologies. Therefore, research is being carried out on a method for grafting OFDMA to the existing synchronous CDMA transmission standard. Research is also being performed to show that OFDMA is higher than CDMA in gain. In order to design the new standard, there is a demand for an optimal grafting method of integrating OFDMA into a CDMA system, maintaining cell planning using a pilot channel supporting the existing cellular mobile communication system.
As described above, the OFDMA scheme is a combined multiple access scheme of frequency division and time division, and is advantageous in that it is robust against multipath fading and has excellent frequency efficiency. If the OFDMA system having the foregoing advantages is integrated into the existing CDMA system, compatibility is not provided and interference occurs due to a difference between their transmission technologies.
In order to solve this problem, 3rd Generation Partnership Project (3GPP), an asynchronous CDMA standardization organization, considers a method for forming a forward link by allocating other frequency channels to the OFDMA system in addition to the existing CDMA system. However, such a method has a problem in that radio resources should be additionally allocated for the OFDMA system. Accordingly, there is a demand for a new mobile communication standard for providing compatibility between different communication technologies by using the advantages of the existing CDMA scheme and the OFDM scheme.